For the new study, Dr. Erin LeBlanc from the Kaiser Permanente Center for Health Research in Portland, Oregon, and her colleagues chemically analyzed pills from 15 vitamin D bottles purchased at local stores and two doses of compounded vitamins.

Supplement bottles were labeled as containing 1,000, 5,000 or 10,000 international units (IU) of vitamin D. Just one-fourth of the vitamins met the standard of all pills falling between 90 and 120 percent of the expected dose, based on a random selection of five pills per bottle.

Pills made by the one manufacturer that was verified by the U.S. Pharmacopeial Convention (USP) - a third-party tester - were all within six percent of the listed dose, LeBlanc and her colleagues found.

"Consumers buying those products can be more assured that what they're getting in their pills is what's labeled," she said.

Compounded vitamin D pills were marked as 1,000 or 50,000 IU. Those high-dose vitamins can be prescribed by doctors and are mixed by pharmacies, unlike the store-bought brands.

One-third of the pills met the slightly stricter dosing standards for compounded vitamins, according to the findings published this week in JAMA Internal Medicine.

Compounding pharmacies have come under the microscope in recent months after a meningitis outbreak was tied to injectable steroids made by the New England Compounding Center in Framingham, Massachusetts.

LOOSE MANUFACTURING STANDARDS

For most adults, the recommended daily allowance of vitamin D is 600 to 800 IU.

However, people who are deficient in vitamin D are sometimes prescribed high doses to boost the amount in their blood.

"I'm a physician, so I've told my patients to take vitamin D, and I guess I always just figured that if they were taking it from a bottle, they would get the amount that was listed on the label," LeBlanc told Reuters Health.

The main concern with such wide variability, she added, is that people with low vitamin D levels who rely on supplements may end up consistently getting less of the vitamin than is labeled in some cases.

Cohen said laws regulating supplements are often not strictly enforced, so companies can get away with spending less on manufacturing standards at the expense of quality and consistency.

He told Reuters Health the best way for consumers to get supplements with the correct dose of ingredients is to purchase brands with a seal from USP or NSF International, another third-party verifier.

After spending four miserable days in the hospital with gallstones, Chicago resident Priscilla Dias Hill resolved to improve her eating habits. Rather than working with a licensed dietitian, however, she chose a holistic health coach.